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Use of Aromatase Inhibitors in Breast Carcinoma Endocrine-Related Cancer (1999) 6 75-92 Use of aromatase inhibitors in breast carcinoma R J Santen and H A Harvey1 Department of Medicine, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908, USA 1Department of Medicine, Penn State College of Medicine, Hershey, Pennsylvania 17033, USA (Requests for offprints should be addressed to R J Santen) Abstract Aromatase, a cytochrome P-450 enzyme that catalyzes the conversion of androgens to estrogens, is the major mechanism of estrogen synthesis in the post-menopausal woman. We review some of the recent scientific advances which shed light on the biologic significance, physiology, expression and regulation of aromatase in breast tissue. Inhibition of aromatase, the terminal step in estrogen biosynthesis, provides a way of treating hormone-dependent breast cancer in older patients. Aminoglutethimide was the first widely used aromatase inhibitor but had several clinical drawbacks. Newer agents are considerably more selective, more potent, less toxic and easier to use in the clinical setting. This article reviews the clinical data supporting the use of the potent, oral competitive aromatase inhibitors anastrozole, letrozole and vorozole and the irreversible inhibitors 4-OH andro- stenedione and exemestane. The more potent compounds inhibit both peripheral and intra-tumoral aromatase. We discuss the evidence supporting the notion that aromatase inhibitors lack cross- resistance with antiestrogens and suggest that the newer, more potent compounds may have a particular application in breast cancer treatment in a setting of adaptive hypersensitivity to estrogens. Currently available aromatase inhibitors are safe and effective in the management of hormone- dependent breast cancer in post-menopausal women failing antiestrogen therapy and should now be used before progestational agents. There is abundant evidence to support testing these compounds as first-line hormonal therapy for metastatic breast cancer as well as part of adjuvant regimens in older patients and quite possibly in chemoprevention trials of breast cancer. Endocrine-Related Cancer (1999) 6 75-92 Introduction concept is that estrogens can be metabolized to catecholestrogens and then to quinones which directly Epithelial cells of the normal breast undergo dramatic damage DNA. These two processes - the estrogen changes during various events in a woman’s life such as receptor-mediated, genomic effects on proliferation and puberty, the follicular and luteal phases of the menstrual the receptor-independent, genotoxic effects of estrogen cycle, pregnancy and menopause. The co-ordinated metabolites - can act either in an additive or synergistic interaction of growth factors and steroid hormones fashion to cause breast cancer (Santen et al. 1999). regulate the proliferation and differentiated function of Breast cancers which arise in patients can be divided epithelial and stromal cells in the normal mammary gland. into two subtypes: those which are dependent upon The key growth factors are insulin-like growth factor-I, hormones for growth and those which grow independently prolactin, insulin, the fibroblast growth factor family of of hormonal stimulation (Santen et al. 1990). In the growth factors and growth hormone, and major steroid hormone-dependent subtype, the role of estrogens as hormones are estradiol, progesterone and testosterone modulators of mitogenesis overrides the influence of other (Frantz & Wilson 1998). factors. These sex steroids stimulate cell proliferation For the process of inducing breast cancer, estrogens directly by increasing the rate of transcription of early appear to play a predominant role. These sex steroids are response genes such as c-myc and indirectly through believed to initiate and promote the process of breast stimulation of growth factors which are produced largely carcinogenesis by enhancing the rate of cell division and in response to estrogenic regulation (Dickson & Lippman reducing time available for DNA repair. An emerging new 1995). Endocrine-Related Cancer (1999) 6 75-92 Online version via http://www.endocrinology.org 1351-0088/99/006-075 © 1999 Society for Endocrinology Printed in Great Britain Downloaded from Bioscientifica.com at 09/24/2021 11:39:59PM via free access Santen and Harvey: Use of aromatase inhibitors in breast carcinoma Based upon the concept that estrogen is the proximate Physiology and regulation of aromatase regulator of cell proliferation, two general strategies were Aromatase is a cytochrome P-450 enzyme which catalyzes developed for treatment of hormone-dependent breast cancer: blockade of estrogen receptor action and inhibition the rate-limiting step in estrogen biosynthesis, the of estradiol biosynthesis. Antiestrogens such as tamoxifen conversion of androgens to estrogens (Simpson et al. bind to the estrogen receptor and interfere with trans- 1997, Sasano & Harada 1998). Two major androgens, cription of estrogen-induced genes involved in regulating androstenedione and testosterone, serve as substrates for aromatase. The aromatase enzyme consists of a complex cell proliferation. Clinical trials showed tamoxifen to be containing a cytochrome P-450 protein as well as the effective in inducing objective tumor regressions and to be flavoprotein NADPH cytochrome P-450 reductase associated with minimal side-effects and toxicity. The (Simpson et al. 1997). The gene coding for the second strategy, blockade of estradiol biosynthesis, was demonstrated to be feasible using the steroidogenesis cytochrome P-450 protein (P-450 AROM) exceeds 70 kb inhibitor, aminoglutethimide, which produced tumor and is the largest of the cytochrome P-450 family regressions equivalent to those observed with tamoxifen (Simpson et al. 1993). The cDNA of the aromatase gene (Santen et al. 1990). However, side-effects from amino- contains 3.4 kb and encodes a polypeptide of 503 amino acids with a molecular weight of 55 kDa. Approximately glutethimide were considerable and its effects on several 30% homology exists with other cytochrome P-450 steroidogenic enzymes required concomitant use of a proteins. Because its overall homology to other members glucocorticoid (Santen et al. 1982). Consequently, of the P-450 superfamily is low, aromatase belongs to a tamoxifen became the preferred, first-line endocrine agent with which to treat advanced breast cancer. However, the separate gene family designated CYP19. clinical efficacy of aminoglutethimde focused attention Recent studies indicate that the transcription of the upon the need to develop more potent, better tolerated, and aromatase gene is highly regulated (Simpson et al. 1989, more specific inhibitors of estrogen biosynthesis. 1993, 1997). The first exon of the aromatase gene is transcribed into aromatase message but not translated into protein. There exist nine alternative first exons which can Inhibition of estradiol biosynthesis initiate the transcription of aromatase. Each of these alternate exons contains upstream DNA sequences which Multiple strategies could be used to inhibit estradiol can either enhance or silence the transcription of arom- biosynthesis as a treatment for estrogen-dependent breast atase. Different tissues utilize specific alternate exons to cancer. Inhibition of several enzymes in the steroidogenic initiate transcription. For example, the placenta utilizes pathway, including cholesterol side-chain cleavage, 3 alternate exon I.1, the testis alternate exon II, adipose beta-ol-dehydrogenase-delta 4-5 isomerase, 17-alpha tissue I.3 and I.4 and brain If. Enhancers which react with hydroxylase, 17-beta hydroxysteroid dehydrogenase, upstream elements of these alternate exons markedly estrone sulfatase, and aromatase, could be used to reduce stimulate the rate of transcription of the aromatase gene. the biosynthesis of estradiol and potentially cause Thus, each tissue can regulate the amount of aromatase hormone-dependent breast tumor regression. An addition- transcribed in a highly specific manner (Simpson et al. al strategy is the use of exogenous glucocorticoid to inhibit 1993). release of adrenocorticotropin (ACTH) and suppress Aromatase expression occurs in many organs, includ- estrogen production. Finally, synthetic progestins such as ing ovary, placenta, hypothalamus, liver, muscle, adipose megestrol acetate and medroxy-progesterone acetate exert tissue, and breast cancer itself. Aromatase catalyzes three glucocorticoid effects and inhibit estradiol synthesis by separate steroid hydroxylations which are involved in the suppressing ACTH. conversion of androstenedione to estrone or testosterone The ideal strategy would be to block the synthesis of to estradiol. The first two give rise to 19-hydroxy and 19- estrogen without inhibiting production of other important aldehyde structures and the third, although still contro- steroids or giving pharmacological amounts of progestins versial, probably also involves the C-19 methyl group with or glucocorticoids. For this reason, blockade of the release of formic acid (Fishman & Hahn 1987). This terminal step in estradiol biosynthesis catalyzed by the enzymatic action results in the saturation of the A-ring of enzyme aromatase is considered a more specific and the steroid molecule to produce an aromatic structure, therefore preferable strategy. Several pharmaceutical hence the term aromatization. companies sought to develop potent aromatase inhibitors In the premenopausal state, the major source of designed to specifically block estrogen biosynthesis with-
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